function externalLinks() 
{
   if (!document.getElementsByTagName) return;
   
   var anchors = document.getElementsByTagName("a");
 
   for (var i=0; i<anchors.length; i++) 
   {
      var anchor = anchors[i];
      if (anchor.getAttribute("href") && anchor.getAttribute("rel") == "external") 
         anchor.target = "_blank";
   }
}

window.onload = externalLinks;